Method for controlling the current pulse supply to an electrostatic precipitator

ABSTRACT

In a method for controlling the current pulse supply to the discharge electrodes of an electrostatic precipitator unit in order to achieve maximum separation of dust from gases conducted between the discharge electrodes and the collecting electrodes of the unit at issue, current pulses (I) with a given pulse current are supplied to the discharge electrodes. The pulse frequency is varied, and instantaneous values (U p , U(I=O), U(I=O+1.6)) corresponding to one another, for the voltage (U) between the discharge electrodes and the collecting electrodes are measured for a number of pulse frequencies. Then, the current pulse supply to the discharge electrodes is set to the pulse frequency at which the highest instantaneous value has been measured.

The present invention relates to a method for controlling, in anelectrostatic precipitator unit with discharge electrodes and collectingelectrodes between which dustladen gases are conducted for dustseparation, the current pulse supplied to the discharge electrodes, inorder to achieve maximum dust separation.

Usually, electrostatic precipitators are made up of a number ofprecipitator units arranged one after another, through which dustladengases are successively conducted in order to be cleaned. Each of theseelectrostatic precipitator units has an inner chamber which is dividedinto a number of parallel gas passages by means of a number of verticalcurtains of earthed steel plates arranged side by side to form thecollecting electrodes of each unit. A number of vertical wires to whicha negative voltage is connected are arranged in each gas passage to formthe discharge electrodes of each unit. Due to corona discharges from thedischarge electrodes, the gases are ionized in the electric field in thegas passages. The negative ions are attracted by the steel plates and,when moving towards these, collide with the dust particles in the gases,such that the particles are charged, whereupon they are separated fromthe gases when they are attracted by the nearest steel plate (collectingelectrode), where they settle and form a growing layer of dust.

Generally, dust separation becomes more efficient as the voltage betweenthe electrodes increases. The voltage should, however, not be too high,since that may cause flash-overs between the electrodes. Too high acurrent per unit area towards the collecting electrode may entail thatthe dust layer is charged faster than it is discharged towards saidcollecting electrode. Then, this charging of the dust layer entailssparking in the layer itself, so-called back-corona, and dust is thrownback into the gas. The risk of back-corona becomes greater as theresistivity of the dust increases.

To reduce the risk of back-corona, especially in separation of dust ofhigh resistivity, and at the same time maintain such a current supply tothe discharge electrodes that corona discharges occur therein, thedischarge electrodes are now usually supplied with current pulses. Eachprecipitator unit has a separate, controllable current and/or voltagesupplying circuit associated with control equipment, such that thecurrent and/or voltage supplied to each unit can be separatelycontrolled. Thus, the current supplied to the discharge electrodes ofeach unit is separately adjusted in such a manner that maximum dustseparation is obtained. Today, such an adjustment is carried outentirely by hand in that the current pulse supply is adjusted and thealteration caused thereby of the degree of dust separation is controlledby measuring the opacity of the gases from the electrostaticprecipitator. This adjustment is repeated until a lowest opacity valuehas been obtained. This method is, however, time-consuming andfurthermore requires that the operator be specially trained and havegreat experience in electrostatic precipitators, since a considerabledegree of "feeling" is needed to be able to decide which otherparameters may possibly have influenced the opacity measuring during thesetting operation. Furthermore, considerable adjustments have to be madefor an efficient use of the opacity measurings.

SUMMARY AND OBJECTS OF THE PRESENT INVENTION

Therefore, the object of the present invention is to provide a simplecurrent supply control method having none of the above disadvantages.

This object is achieved by a method where current pulses with a givenpulse current are supplied to the discharge electrodes, that the pulsefrequency is varried, that instantaneous values corresponding to oneanother, for the voltage between the discharge electrodes and thecollecting electrodes are measured for a number of different pulsefrequencies, and that the current pulse supply to the dischargeelectrodes is then set to the pulse frequency at which the greatestinstantaneous value has been measured.

In a preferred embodiment, the peak value of the voltage is measured forevery pulse frequency.

In another preferred embodiment, the instantaneous value of the voltageat the end of the current pulse is measured for every pulse frequency.

In yet another preferred embodiment, the instantaneous value of thevoltage at a predetermined moment after the current pulse has ended, butbefore the following current pulse has started is measured for everypulse frequency. In this connection, the instantaneous value of thevoltage, for example, 1.6 ms after the current pulse has ended ismeasured for every pulse frequency.

Preferably, the discharge electrodes are supplied with current pulsesfor which the pulse current is set to a maximum value considering thecapacity of the current supply circuit of the unit and/or consideringany flashovers between the discharge electrodes and the collectingelectrodes.

The invention will be described in more detail below, reference beinghad to the accompanying drawing, in which

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the relationship between secondary current andsecondary voltage, and the definition of certain parameters;

FIG. 2 corresponds to FIG. 1 and illustrates the relationship betweensecondary current and secondary voltage when dust of low resistivity isseparated, the relationship being also illustrated at lower pulsefrequency;

FIG. 3 corresponds to FIG. 1 and illustrates the relationship betweensecondary current and secondary voltage when dust of high resistivity isseparated, the relationship being also illustrated at lower pulsefrequency.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the relationship between the secondary current I andthe secondary voltage U; i.e., the current and the voltage which occurat the secondary side of a transformer full-wave rectifier device. Theis connected to a alternating voltage which is applied to theelectrostatic precipitator unit. The current level is adjusted bythyristors at the primary side of the device. The thyristors as shown inFIG. 1, when the distance between the current peaks is 10 ms, areignited for every half cycle (CR=1) of the AC voltage. For instance, thethyristors may also be ignited for every third, every fifth, everyseventh, etc. half cycle, which is designated CR=3, CR=5, CR=7 etc.,where CR means "charging ratio". Thus, an increasing CR entails adecreasing pulse frequency. It should be pointed out that therelationship between secondary current and secondary voltage depends onthe degree of back-corona.

FIG. 1 also defines certain parameters used in the followingdescription. Thus, U_(p) designates the peak value of the secondaryvoltage, U(I=O) designates the secondary voltage at the end of thecurrent pulse, and U=(I=O+1.6) designates the secondary voltage 1.6 msafter the current pulse has ended, i.e. at a moment when the secondarycurrent still is zero.

FIG. 2 corresponds to FIG. 1 and illustrates the relationship betweenthe secondary current I and the secondary voltage U when dust of lowresistivity is separated. In addition to what is shown in FIG. 1, FIG. 2illustrates, by means of a dashed line, the secondary voltage obtainedat lower pulse frequency (CR>1), and it is apparent that the secondaryvoltage is lower over the whole cycle when the pulse frequency is lower.

FIG. 3 corresponds to FIG. 1 and illustrates the relationship betweenthe secondary current I and the secondary voltage U when dust ofsufficient resistivity to produce back-corona is separated. In additionto what is shown in FIG. 1, FIG. 3 illustrates, by means of a dashedline, the secondary voltage obtained at lower pulse frequency (CR>1),and it is apparent that the secondary voltage at lower pulse frequencybecomes lower at the beginning of the current pulse, but rapidlyincreases to transcend the continuous voltage curve after a certaintime.

A test was made with an electrostatic precipitator having two successiveunits for cleaning of flue gases from a black liquor recovery boiler, inwhich MgO of very high resistivity was separated from the flue gases.The pulse current and the pulse frequency for the first unit were keptconstant at values resulting in an efficient separation of MgO. Thepulse frequency for the second unit was varied for a number of differentpulse current values, and the opacity of the flue gases from the unitwas measured for different CR values. The CR value at which the opacitywas at its lowest; i.e., at which the separation was at it highest; wasnoted. At the pulse current values, U_(p), U(I=0) and U(I=0+1.6) fordifferent CR values were also measured, and the CR value for which thevoltage U_(p), U(I=0) and U(I=0+1.6), respectively, was highest, wasnoted. When these noted CR values were compared, the CR value at whichU(I=0+1.6) was highest, was found to agree with the CR value at whichthe opacity was at its lowest.

Another test was made with an electrostatic precipitator for cleaning offlue gases from a coal-fired power station, in which ash of lowresistivity was separated from the flue gases. In this case, the CRvalue at which U_(p) was highest, was found to be closest to the CRvalue at which the opacity was at its lowest. However, the CR values atwhich U(I=0) and U(I=0+1.6) were highest, also agreed with the CR valueat which the opacity was at its lowest.

Furthermore, another test was also made with an electrostaticprecipitator for cleaning of flue gases from a coal-fired power station,in which ash with high resistivity was separated from the flue gases. Inthis case, the CR values at which all voltages U_(p), U(I=0) andU(I=0+1.6) were highest, agreed with the CR value for which the opacitywas at its lowest.

Thus, there is a relationship between the secondary voltage and theseparation capacity. For a given pulse current, obtained for instancewith a predetermined ignition angle for the thyristors at the primaryside of the transformer full-wave recitifer device, it was found thatthe CR values at which U_(p), U(I=0) and U(I=0+1.6) are highest, gave apulse frequency setting very close to the setting resulting in maximumseparation. A CR value at which U_(p) is highest, is preferable whendust of low resistivity is separated, and a CR value at which U(I=0+1.6)is highest, is preferable when dust of high resistivity is separated. Ofthe chosen parameters U_(p), U(I=0) and U(I=0+1.6), none seems to bemore suitable than the others under all types of separation conditions.It is also conceivable to use as a parameter some kind of average valuefor the secondary voltage, the value being centered upon the end pointof the current pulse or any other suitable point. It should be observedthat the parameter U(I=0+1.6) is rather abitrarily chosen, and that thesecondary voltage at any other suitable moment between two successivecurrent pulses also can be used as a parameter.

On the basis of the teachings related above, the adjustment of thecurrent supply to the discharge electrodes of an electrostaticprecipitator unit is thus suitably carried out in accordance with theinvention as follows. The discharge electrodes of the electrostaticprecipitator unit is supplied with current pulses for which the pulsecurrent is set to a maximum value considering the capacity of thecurrent supply device of the unit and/or considering any flash-oversbetween the discharge electrodes and the collecting electrodes. For theother units possibly forming part of the same electrostaticprecipitator, the pulse current and pulse frequency are, during thisoperation, maintained at constant values appearing to result inefficient dust separation. The pulse frequency of the current pulses tothe discharge electrodes of the studied unit is varied, and theinstantaneous value of a secondary voltage parameter, suitably one ofthe above-mentioned parameters U_(p), U(I=0) and U(I=0+1.6), is measuredfor a number of different pulse frequencies. The current pulse supply tothe discharge electrodes of the studied unit is then set to the pulsefrequency at which the instantaneous value of the checked parameter isat its highest. As mentioned above, this pulse frequency is very closeto the pulse frequency resulting in maximum separation.

As is seen, this setting method, in which separate setting for the unitsin an electrostatic precipitator is possible, is easily carried out andrequires no specialist competence of the operator. Furthermore, themethod gives a rapid response since only electrical signals are used andno measuring of the opacity is needed. The influence caused by evensmall changes of the pulse frequency on the separation capacity of theunit can be controlled by supervision of the chosen secondary voltageparameter. Also, the method should make possible the development ofefficient algorithms for rectifier control.

I claim:
 1. A method for controlling, in an electrostatic precipitatorunit having discharge electrodes and collecting electrodes, a currentpulse supplied to the discharge electrodes, comprising the steps of:(a)supplying current pulses of a non-varying predetermined magnitude to thedischarge electrodes; (b) varying a frequency of the current pulsesupplied in said step (a); (c) measuring an instantaneous voltage valuecorresponding to a voltage between the discharge electrodes and thecollecting electrodes for each different frequency created by said step(b); and (d) supplying current pulses to the discharge electrodes at thefrequency having a maximum instantaneous voltage value measured in saidstep (c).
 2. The method as claimed in claim 1, wherein the instantaneousvoltage value measured for every frequency is a peak value of thevoltage.
 3. The method as claimed in claim 1, wherein the instantaneousvoltage value measured for every frequency is a voltage at an end of acurrent pulse.
 4. The method as claimed in claim 1, wherein theinstantaneous voltage value measured for every frequency is a voltage atan instant of time between an end of one current pulse and a start of anext current pulse.
 5. The method as claimed in claim 4, wherein theinstantaneous voltage value measured for every frequency is a voltageoccurring 1.6 ms after the end of a current pulse.
 6. The method asclaimed in any one of claims 1-5, wherein the discharge electrodes aresupplied with current pulses which are set at a value not exceeding acapacity of a current supply unit of the precipitator and wherein thevalue also prevents flash-overs between the discharge electrodes and thecollecting electrodes.